ABSTRACT\ud Geologic or “deep time” is important for understanding many geologic processes. There are two aspects to deep time. First, events in Earth’s history can be placed in temporal order on an immense time scale (succession). Second, rates of geologic processes vary significantly. Thus, some events and processes require time periods (durations) that are outside a human lifetime by many orders of magnitude. Previous research has demonstrated that learners of all ages and many teachers have poor conceptions of succession and duration in deep time. The question is why. \ud This exploratory, qualitative study investigates the viability of a model (a deep time stool) to capture the underlying factors necessary for a concept of deep time. The model posits that a concept of deep time rests upon: an understanding of succession and duration in conventional time; a robust understanding of large numbers and the proportional relationships among numbers of various magnitudes; and a learner’s geoscience content knowledge. While all three factors may not exist to the same degree in any one individual, all must be present to support a conception of deep time.\ud Thirty-five students in the United States participated in individual task-based interviews: 12 eighth and 11 eleventh graders from a public charter school in the U.S. and 12 university students from two institutions enrolled in an introductory geoscience course. Tasks and questions probed students’ understandings of the three factors within and outside a deep time context, and the study is unique for that reason.\ud Results indicate all three factors play an important role in how students understand deep time. While succession in conventional time proved non-problematic, duration was more difficult for participants. Some students were confused about the relationships among numbers in the thousands and millions, and others appeared to have little understanding of time periods up to 100 years. Participants had just as much difficulty dealing with the duration for events in conventional time as they did for those in deep time if the events were unfamiliar to them. \ud Time and number share a similar spatial mapping strategy while knowledge of large numbers and geoscience content knowledge appear to provide reference points that can be used to judge the temporal order or duration of geoscience events. Implications for future research and classroom practice are discussed.\u
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